Thermally regulated heating elements of a wide variety of types have existed for some time. Most often these elements have utilized some form of feedback control system in which the temperature produced is sensed and the source of electrical energization to the heating element is controlled either in a continuous, proportional or step-wise switching fashion to achieve more-or-less constant temperature. Utilizing a wide variety of thermal sensors and various control systems, these approaches continue to be successfully used in many applications.
However, there are many situations requiring temperature regulation which the prior art feedback control systems are not capable of handling adequately.
One of these situations involves differential thermal loading of the heating element over its extent, such that its various parts operate at different temperatures. In order to satisfactorily regulate temperature under such a loading condition with the prior art feedback control systems, the heating element must be subdivided into a plurality of smaller heating elements and each one must be provided with independent sensing means and feedback control, etc. In general, this approach is far too clumsy, unreliable and expensive.
A second situation in which the prior art feedback control systems are not adequate is where the heating element itself is so small as to make adequate monitoring of its temperature by a separate sensing means impractical. In some instances it has been possible to cope with these situations by utilizing a thermally dependent parameter of the heating element as a means of sensing its own temperature. For example, it is possible in some instances to energize a heating element in a pulsed manner and sense the resistance of the heating element during the portion of the power supply cycle when it is not energized. If the cycle of alternate energization and temperature sensing is made short in comparison to the thermal time constants of the heating element and its load, such a scheme can be used to alter the duty cycle of energization by means of a feedback control system to produce a constant temperature. However, the resultant apparatus is complex and relatively expensive.
Another instance in which traditional means of feedback temperature control is inappropirate occurs when the thermal time constants associated with the heating element and thermal load are so short that they exceed the speed of response of the thermal sensor and the control system. Typically these situations arise when the heating element is extremely small but can also occur in heating elements of great extent but low mass such as in a long filamentary heater.
The above and many other difficult thermal regulation problems could be reliably, simply and inexpensively solved if there were an electrically resistive heating element which provided adequate intrinsic self-regulation of temperature despite changes in thermal load.